Voltage regulators, such as DC to DC converters, are used to provide stable voltage sources for electronic systems. Efficient DC to DC converters are particularly needed for power management in low power devices, such as laptop notebooks and cellular phones. Switching voltage regulators (or simply “switching regulators”) are known to be an efficient type of DC to DC converter. A switching regulator generates an output voltage by converting an input DC voltage into a high frequency voltage, and filtering the high frequency input voltage to generate the output DC voltage. Specifically, the switching regulator includes a switch for alternately coupling and decoupling an input DC voltage source, such as a battery, to a load, such as an integrated circuit. An output filter, typically including an inductor and a capacitor, is coupled between the input voltage source and the load to filter the output of the switch and thus provide the output DC voltage. A controller, such as a pulse width modulator or a pulse frequency modulator, controls the switch to maintain a substantially constant output DC voltage.
LDMOS (laterally diffused metal oxide semiconductor) transistors are used in switching regulators as a result of their specific on-resistance and drain-to-source breakdown voltage.
Production testing of LDMOS transistors involves ensuring that the on-resistance is appropriately low so that the operation of the device will be efficient. In addition, production testing involves ensuring that the leakage current (i.e. the current through the device when it is off) is not too high. High leakage current can be symptomatic of defects in the transistor. These defects might prevent the transistor from working correctly, or might cause the transistor to be unreliable over time.
Testing the leakage current of an LDMOS device generally involves keeping the device switched off, applying a voltage from the drain to the source, and measuring the resulting leakage current flowing from drain to source. The leakage current should measure within an expected range, in accordance with the design of the transistor. For example, a transistor whose voltage rating is lower will have a higher expected leakage current than an otherwise similar transistor with a higher voltage rating. Given all the design attributes of a transistor, a manufacturer can determine a bounded range of acceptable values for leakage current, outside of which a device should be deemed “defective” and should be discarded.